Maximum Alternating Subsequence Sum Solution

Maximum Alternating Subsequence Sum

The alternating sum of a 0-indexed array is defined as the sum of the elements at even indices minus the sum of the elements at odd indices.

• For example, the alternating sum of [4,2,5,3] is (4 + 5) - (2 + 3) = 4.

Given an array nums, return the maximum alternating sum of any subsequence of nums (after reindexing the elements of the subsequence).

A subsequence of an array is a new array generated from the original array by deleting some elements (possibly none) without changing the remaining elements’ relative order. For example, [2,7,4] is a subsequence of [4,2,3,7,2,1,4] (the underlined elements), while [2,4,2] is not.

Example 1:

Input: nums = [4,2,5,3]
Output: 7
Explanation: It is optimal to choose the subsequence [4,2,5] with alternating sum (4 + 5) - 2 = 7.

Example 2:

Input: nums = [5,6,7,8]
Output: 8
Explanation: It is optimal to choose the subsequence [8] with alternating sum 8.

Example 3:

Input: nums = [6,2,1,2,4,5]
Output: 10
Explanation: It is optimal to choose the subsequence [6,1,5] with alternating sum (6 + 5) - 1 = 10.

Constraints:

• 1 <= nums.length <= 105
• 1 <= nums[i] <= 105

Program:

class Solution:
    def maxAlternatingSum(self, nums: List[int]) -> int:
        ans = 0
        direction = 'down'
        n = len(nums)
        for i in range(n-1):
            if direction == 'down' and nums[i] >= nums[i+1]:
                ans += nums[i]
                direction = 'up'
            elif direction == 'up' and nums[i] <= nums[i+1]:
                ans -= nums[i]
                direction = 'down'
        if direction == 'up':
            return ans
        return ans + nums[-1]

Credit : Here

Weekly Contest 247

• Maximum Product Difference Between Two Pairs
• Cyclically Rotating a Grid
• Number of Wonderful Substrings
• Count Ways to Build Rooms in an Ant Colony

Biweekly Contest 55

• Remove One Element to Make the Array Strictly Increasing
• Remove All Occurrences of a Substring
• Maximum Alternating Subsequence Sum
• Design Movie Rental System

June Long Challenge 2021 Solutions

• Maximum Frequent Subarray Sum solution codechef
• Dual Distance solution codechef
• Optimal Xor Set solution codechef
• Minimum Subtree Cover solution codechef
• Minimum Dual Area solution codechef
• Bitwise Tuples solution codechef
• Shortest Route solution codechef
• Bella ciao solution codechef
• Summer Heat solution codechef